Melt the butter in a soup pot. Saute the onions until soft, about 5 minutes. Add the garlic and cook for about a minute. Add the lettuce and stir frequently until it wilts. Add the broth, water, peas, marjoram, salt, and pepper. Simmer uncovered for 10 minutes. Puree the soup in a blender making sure to leave an opening covered with a hand towel for the steam to escape. Return the soup to a clean pot. Whisk in the sour cream and heat just until heated through. Don’t boil it at this point. Serve with a garnish of pea shoots or mint.